1. Field of Invention
The present invention relates to methods for manufacturing semiconductor devices, and more particularly, relates to a semiconductor device manufacturing method including an etching process or a surface-treating process which exhibits high etching selectivity for the subject layer being etched as against the resist film (layer) and the base layer while achieving a high tolerance to plasma (plasma tolerance).
2. Description of Related Art
Nowadays, the manufacture of semiconductor devices in increasingly higher degrees of integration requires an etching process capable of forming highly accurate fine patterns with a high etching selectivity.
For forming highly accurate patterns in semiconductor devices, an important factor is the accuracy in forming a mask in a photolithographic process which is performed using a photoengraving technique prior to the etching process. In such a photolithographic process, since the focal depth will necessarily be shallower for a finer pattern, a thinner resist film should essentially be formed. The thickness of such a resist film, however, cannot be uniformly predetermined, since the resist film is also etched to some extent and the thickness of the resist film is reduced in the etching process, and more specifically, the etching reaction proceeds while the etched ingredients from the resist film adhere to the side walls that form in the etched portions of the layer subject to etching. As a result, the shape of the etched patterns can be maintained. When the initial thickness of the resist film is close to the thickness reduced by etching, the resist film ingredients adhering to the side walls disappear, and the edges of the etched portions are etched to be round, namely, pattern breakdown occurs. Since the protection of such side walls is more significant for finer patterns, the thickness of the resist film is on a trade-off relationship between the accuracy in the photolithographic process and the pattern-shape maintenance in the etching process. In other words, success in highly accurate processing depends on the degree of improvement in the etching selectivity for the subject layer as against the resist film (anti-resist selectivity).
Further, in the etching process, the achievement of a high selectivity for the subject layer against the base layer (anti-base-layer selectivity) is essential in order to enhance the performance and reliability of the manufactured semiconductor device. For example, in gate etching, a high selectivity for the subject layer against a gate oxide film should be secured. Similarly, in contact-hole etching, the selectivity for the subject layer should be high against, for example, Si in a substrate, a silicide layer, and a silicon nitride stopper layer comprising Si.sub.3 N.sub.4, SiN.sub.x or the like formed as a gate side wall or formed on a gate or a side wall in a self-alignment contact process. Further, in via-hole etching, such a selectivity should be high against an underlying metal such as TiN or Ti.sub.2 N of an antireflection film.
In addition, in the etching process, vertical etching is not always most desirable. For example, a wiring pattern should preferably be etched in a normal taper manner in order to improve the coverage of an interlayer insulating film, and also, a contact-hole or via-hole should be etched in a normal taper manner in order to improve the coverage of a metallic wiring pattern in the hole.
Hitherto, investigations have been made on the main etching gas or additional gas in order to achieve high anti-resist and anti-base-layer selectivity, and to control the degree of taper in the etched pattern shape. For satisfying the above-described requirements concerning etching, for example, in an oxide-film etching process using a C-F-based gas such as CF.sub.4 or C.sub.3 F.sub.8, the C/F ratio should be preferably raised. Since F serving as a main etchant is capable of etching even the base layer, the base layer should preferably be covered with a protective film after the etch-off for the film which should be etched. By raising the C/F ratio, a number of CF.sub.2 radicals and CF radicals, which can be precursors for a polymer film, can be generated in the plasma, and a fluorocarbon protective film can be thereby readily formed on the base layer surface. For this reason, in oxide-film etching, straight-chain fluorocarbons having a large number of carbon atoms such as C.sub.2 F.sub.6 and C.sub.3 F.sub.8, or unsaturated fluorocarbons such as C.sub.4 F.sub.8 have been used as a main etching gas having a C/F ratio higher than CF.sub.4. Meanwhile, the object of the addition of an additional gas is to scavenge excessive F in the plasma and to thereby raise the C/F ratio in the plasma. In this view, hydrogen-containing gases such as H.sub.2, CHF.sub.3, CH.sub.2 F.sub.2 and CH.sub.3 F, CO, and others have been used as an additional gas.
Conventional etching processes for manufacturing semiconductor devices, in which CF.sub.2 radicals and/or CF radicals are generated, are allowed to adhere and are polymerized, however, have some problems as follows.
First of all, it is difficult to decompose a C.sub.x F.sub.y gas to efficiently obtain CF.sub.2 radicals and/or CF radicals. Etching is performed in various manners of various discharging types, such as RIE (Reactive Ion Etching), MERIE (Magnetron Enhanced Reactive Ion Etching), ECR (Electron Cyclotron Resonance) etching and helicon wave etching, and the plasma electron energy level in one discharging type is different from that in another type. Meanwhile, a compound molecule has a specific dissociation energy. Accordingly, the etchant gas must be selected in accordance with the type of the etching system.
Further, although a gas system composed only by considering generation and polymerization of CF.sub.2 radicals and/or CF radicals may satisfy the requirements concerning the etching selectivity, it is actually accompanied by problems such as a lowered etching rate, difficulty in removal of the resist film after etching, and an increased contact resistance due to carbon implantation into the base layer in a hole-forming process. For example, when a gas having a high C/F ratio is used, the etching rate is reduced due to the generation of a polymer film on the etched surface. If the discharging power is increased in order to compensate for such reduction, dissociation of CF.sub.2 radicals and/or CF radicals further progresses to increase fluorine atoms. As a result, although the etching rate can be raised, the anti-base-layer selectivity is sacrificed. Accordingly, a high anti-resist or anti-base-layer selectivity, and high level requirements on other properties can rarely be achieved or satisfied at the same time by using such a conventional gas system composed only by considering generation and polymerization of CF.sub.2 and/or CF radicals.
Secondly, a sufficient etching selectivity can rarely be achieved according to polymerization of CF.sub.2 radicals and/or CF radicals due to their slow polymerization rate and the low tolerance of the resulting polymer film against plasma.
In conventional CF.sub.2 - and CF-radical polymerization, each CF.sub.2 or CF radical is successively bonded with one another on the layer surface. Accordingly, a long time period is required until the layer surface is covered with the resulting polymer film, and in the meantime, the etching reaction further proceeds to etch the non-covered portions of the base layer. Further, such a slow polymerization rate includes a slow polymerization rate in the direction of the polymer film thickness. Accordingly, even if the base layer surface is once covered with the resulting polymer film, the polymer film will be immediately broken since the polymer film does not yet have a thickness tolerable to ion impact.
Further, since the polymer film is basically free of intramolecular polarity and the bonding strength between polymer molecules consists only of the intermolecular force, a high plasma tolerance can be enhanced only by thickening the polymer film. In summary, a sufficient etching selectivity cannot be achieved due to the slow polymerization rate and the low plasma tolerance of the polymer film.
Moreover, polymerization of CF.sub.2 radicals and/or CF radicals adhering to the layer surface requires an energy supplementation by ion impact. The energy level required for such supplementation is, however, lower than that of the bias voltage required for improvement of the etching rate. Due to this, if the bias voltage is raised in order to improve the etching rate, CF.sub.2 radicals, CF radicals and a polymer film thereof leave the layer surface due to excessive ion impact.
Furthermore, during the etching process, the fluorocarbon in the original etchant gas, in which the number of carbon atoms is large for achieving a higher C/F ratio, is decomposed into molecules having small numbers of carbon atoms in order to obtain CF.sub.2 radicals and CF radicals. As a result, thus-generated carbon-containing ions having small molecular weights may be directly implanted into the base layer from the surface, or CF.sub.2 radicals and CF radicals adhering to the layer surface before being polymerized may indirectly be implanted into the base layer by ion impact added from above. Particularly in contact-hole etching, a carbon-implanted layer formed according to such a mechanism can be a cause of high electric resistance. Although such a carbon-implanted layer has been conventionally removed in a post-treatment such as chemical dry etching (CDE), carbon implantation should be restricted to a level as low as possible in the main etching step since the depth of a junction in a future semiconductor device will be reduced to a level below the range controllable by CDE. Additionally, in a via-hole, the electric resistance increases in response to an increase in the amount of carbon implanted into an underlying metal such as TiN of an antireflection film, and which is also a problem with the conventional etching process in which CF.sub.2 radicals and CF radicals are generated, allowed to adhere, and polymerized.
Thirdly, conventional gas systems cause some problems concerning stability in apparatus operation.
As described above, a C.sub.x F.sub.y polymer film formed according to the polymerization of CF.sub.2 radicals and/or CF radicals is basically free of intramolecular polarity, and the bonding strength between polymer molecules consists only of the intermolecular force. Accordingly, during a continuous operation, when such radicals adhere to the inner surface of a processing chamber to form a fluorocarbon film and the film then accumulates to some extent, the film starts peeling off from the surface due to the stress of the film itself or the stress induced by the heat cycle generated by a discharge cycle, and then scatters particles. Such particles generate pattern deficiencies in the semiconductor devices, and readily lower the yield of the products.
Further, since an electrostatic chuck is mainly employed as a wafer-holding mechanism in an etching apparatus because of its ability to achieve high uniformity in a wafer surface, foreign substances such as particles peeled from the chamber wall that enter between the wafer-holding stage of an etching apparatus and a wafer may damage the stage surface by causing abnormal discharge at the back surface of the wafer when a voltage for holding the wafer is applied.
In the case where a conventional gas system is used, chamber cleaning must be frequently performed for the prevention of the above-described problems, which results in a lowered capacity of the apparatus.
Fourthly, the case where a certain gas is used, and more specifically CO is used, is accompanied by some problems.
A so-called self-alignment contact technique is employed for achieving high degrees of integration of semiconductor devices, in which a stopper layer such as a SiN layer is provided on a gate or a side wall, and the contact-opening size at the time of a contact-photolithography can thereby be large and the contact can extend to the gate. In such a self-alignment contact technique, a C.sub.4 F.sub.8 /CO gas mixture is typically used since it achieves a high selectivity for the etching subject (the material to be etched) against SiN. In factories, however, since CO is fed from a storage facility through a central supplying system, the use of CO requires a fairly large amount of labors and costs for safety management.